A polycarbonate is an engineering plastics which is excellent in transparency, heat resistance or impact resistance, and is widely used at the present time in a variety of fields of electric/electronics, automobiles, optical parts, etc. and in other industrial fields.
As a process for producing the polycarbonate, various techniques are investigated, and there have heretofore been widely known a process in which an aromatic dihydroxyl compound is brought into direct reaction with phosgene (interfacial polycondensation process) and a process in which an aromatic dihydroxyl compound and a carbonic acid diester are subjected to transesterification in a molten state (melting polymerization process).
With regard to the interfacial polycondensation process in the processes for producing a polycarbonate, polymerization is carried out, while the resulting polymer is dissolved in a solvent, and therefore the viscosity of the solution is extremely increased when it is intended to produce an aromatic polycarbonate having a high degree of polymerization. Thus, much labor and a long time are required for the purification of the resultant polymer including cleaning, neutralization and the like; and besides the purification is prone to become insufficient, and the concentranted polymer solution is made pasty, thereby making it difficult to deal with, if the solvent is only removed by a conventional method such as heating or pressure reduction at the time of producing a polymer in solid form from the resultant polymer solution. The above-mentioned process, therefore, suffers a number of disadvantages, for example, there is required a complicated procedure of removing the solvent and precipitant which remain in the polymer after the polymer is precipitated by the addition of the precipitant. (Refer to Japanese Patent Publication No. 67394/1976 (Sho 51).)
On the other hand, the transesterification process (melting polymerization process) is advantageous in that it is free from the aforesaid problems with the interfacial polycondensation process, and is capable of producing a polycarbonate at a cost lower than that in the interfacial polycondensation process.
Nevertheless, the transesterification process suffers a serious drawback that the polycarbonate obtained thereby is inevitably colored because of the reaction for a long time at a temperature as high as 280 to 310.degree. C. in usual cases. In addition, the process involves the problem that the polycarbonate obtained thereby is poor in resistance to hydrolysis as a result of the frequent use of a basic catalyst.
There are proposed, in order to solve the problems with the transesterification process (melting polymerization process), for example, a method in which a specific catalyst is employed (refer to Japanese Patent Publication No. 39972/1986 (Sho 61), Japanese Patent Application Laid-Open No. 223036/1988 (Sho 63), etc., a method in which an antioxidant is added to the reactant in the latter period of the reaction (refer to Japanese Patent Application Laid-Open Nos. 151236/1986 (Sho 61), 158719/1987 (Sho 62), etc., a method in which neutralization is performed with an acid such as p-toluenesulfonic acid and the excess acid is neutralized with an epoxy compound (refer to Japanese Patent Publication No. 175368/1992 (Hei 4), etc. However, the problem is not yet sufficiently solved.
Moreover, in order to solve the problem with the transesterification process (melting polymerization process) that this process fails to produce a high molecular weight product, there is proposed the use, in the latter period of the reaction, of a twin screw vent-type kneading extruder (refer to Japanese Patent Application Laid-Open No. 62522/1986 (Sho 61)) or a horizontal stirred polymerization vessel (refer to Japanese Patent Application Laid-Open No. 153925/1990 (Hei 2)). However, the problem is not yet sufficiently solved. There is also proposed a solid-phase polymerization process (refer to Japanese Patent Application Laid-Open No. 158033/1989 (Hei 1), Japanese Patent Publication No. 99553/1994 (Hei 6), etc.), which however, involve the problem that the process fails to improve the deterioration of the product quality due to the residual catalyst containing zinc acetate, tin acetate or the like to be used as a catalyst, and that, in the case where the polymerization is carried out in the absence of catalyst for the purpose of maintaining favorable quality of the product, the rate of reaction is extremely lowered.